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Abstract

In this letter, we investigated the structural and electrical characteristics of high-κ Er2O3 and Er2TiO5 gate dielectrics on the amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistor
(TFT) devices. Compared with the Er2O3 dielectric, the a-IGZO TFT device incorporating an Er2TiO5 gate dielectric exhibited a low threshold voltage of 0.39 V, a high field-effect
mobility of 8.8 cm2/Vs, a small subthreshold swing of 143 mV/decade, and a high Ion/Ioff current ratio of 4.23 × 107, presumably because of the reduction in the oxygen vacancies and the formation of
the smooth surface roughness as a result of the incorporation of Ti into the Er2TiO5 film. Furthermore, the reliability of voltage stress can be improved using an Er2TiO5 gate dielectric.

Keywords:

Amorphous InGaZnO; Thin-film transistor; Er2O3; Er2TiO5

Background

Amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs) are being
extensively explored as a replacement for amorphous and polycrystalline silicon TFTs
in large-area display technologies, such as active-matrix liquid crystal display devices
and active-matrix organic light-emitting displays
[1]. This is due to their high field-effect mobility, low leakage current, excellent
optoelectronic characteristics, good uniformity and stability, and low temperature
fabrication
[2].

To achieve a high drive current at a low gate voltage, we can either employ high-κ materials or thinner gate dielectrics
[3]. However, the decrease in the thickness of gate dielectric is limited due to the
occurrence of electron tunneling. Consequently, high-κ gate dielectric materials, including Al2O3[4], ZrO2[3], Y2O3[5], and HfO2[6], have been studied to reduce the electron tunneling and maintain the large capacitance.
However, HfO2 dielectric film has a critical disadvantage of high charge trap density between the
gate electrode and gate dielectric, as well as the gate dielectric and channel layer
[7]. Recently, rare earth (RE) oxide films have been extensively investigated due to
their probable thermal, physical, and electrical performances
[6]. To date, the application of RE oxide materials as gate dielectrics in a-IGZO TFTs
has not been reported. Among the RE oxide films, an erbium oxide (Er2O3) film can be considered as a gate oxide because of its large dielectric constant
(approximately 14), wide bandgap energy (>5 eV), and high transparency in the visible
range
[8,9]. The main problem when using RE films is moisture absorption, which degrades their
permittivity due to the formation of low-permittivity hydroxides
[10]. The moisture absorption of RE oxide films may be attributed to the oxygen vacancies
in the films
[11]. To solve this problem, the addition of Ti or TiOx (κ = 50 to approximately 110) into the RE dielectric films can result in improved physical
and electrical properties
[12]. In this study, we compared the structural and electrical properties of Er2O3 and Er2TiO5 gate dielectrics on the a-IGZO TFT devices.

Methods

The Er2O3 and Er2TiO5 a-IGZO TFT devices were fabricated on the insulated SiO2/Si substrate. A 50-nm TaN film was deposited on the SiO2 as a bottom gate through a reactive sputtering system. Next, an approximately 45-nm
Er2O3 was deposited by sputtering from an Er target, while an Er2TiO5 thin film (approximately 45 nm) was deposited through cosputtering using both Er
and Ti targets at room temperature. Then, postdeposition annealing was performed using
furnace in O2 ambient for 10 min at 400°C. The a-IGZO channel material (approximately 20 nm) was
deposited at room temperature by sputtering from a ceramic IGZO target (In2O3/Ga2O3/ZnO = 1:1:1). Top Al (50 nm) source/drain electrodes were formed by a thermal evaporation
system. The channel width/length of examined device was 1,000/200 μm. The film structure
and composition of the dielectric films were analyzed using X-ray diffraction (XRD)
and X-ray photoelectron spectroscopy (XPS), respectively. The surface morphology of
the films was investigated by atomic force microscopy (AFM). The capacitance-voltage
(C-V) curves of the Al/Er2O3/TaN and Al/Er2TiO5/TaN devices were measured using a HP4284 LCR meter. The electrical characteristics
of the a-IGZO TFT device were performed at room temperature using a semiconductor
parameter Hewlett-Packard (HP) 4156C (Palo Alto, CA, USA). The threshold voltage (VTH) was determined by linearly fitting the square root of the drain current versus the
gate voltage curve. Field-effect mobility (μFE) is derived from the maximum transconductance.

Results and discussion

Figure
1 displays the XRD patterns of the Er2O3 and Er2TiO5 thin films deposited on the TaN/SiO2/Si substrate. A strong Er2O3 (400) and weak TaN (101) peaks appeared in the Er2O3 film, while only TaN (101) reflection peak was presented in the Er2TiO5 film, revealing that Er2TiO5 thin film was amorphous. The insets (a) and (b) of Figure
1 depict the AFM images of the Er2O3 and Er2TiO5 thin films, respectively. The Er2O3 sample shows a higher surface roughness compared with the Er2TiO5 sample. This is attributed to the increase in the growth of the grain size, which
is consistent with the XRD result. Another cause for a rough surface is the nonuniform
volume expansion of Er2O3 film because of the nonuniform moisture absorption of the film
[10].

Figure
2a,b presents the Er 4d5/2 and O 1s XPS spectra of the Er2O3 and Er2TiO5 dielectric films, respectively. In the three sets of spectra, each fitting peak is
assumed to follow the general shape of the Lorentzian-Gaussian function: one peak
represents the Er-OH bonds (located at 170.4 eV), the second the Er-O-Ti bonds (located
at 169.9 eV), and the third the Er-O bonds (located at 168.4 eV)
[13]. The Er 4d5/2 peak of the Er2O3 film has two intensity peaks corresponding to Er2O3 and Er(OH)x. For the Er2TiO5 film, the intensity of Er 4d5/2 peak corresponding to Er2TiO5 was larger than that of Er2O3. Furthermore, the Er 4d5/2 peak corresponding to Er2O3 for Er2TiO5 sample had a lower intensity compared with Er2O3 sample. These results are due to the reaction of TiOx with the Er atom to form an Er2TiO5 structure. The O 1s spectra of the Er2O3 and Er2TiO5 films are shown in Figure
2b with their appropriate peak curve-fitting lines. The O 1s signal comprised three peaks at 530.2, 531, and 532.7 eV, which we assign to Er2O3[14], Er2OTi5, and Er(OH)x, respectively. The intensity of O 1s peak corresponding to Er(OH)x bonding for the Er2O3 film was larger in comparison with the Er2TiO5 film, indicating that the reaction between the Er and water caused hydroxide units
in the film. The O 1s peak of the Er2TiO5 film exhibits a large intensity peak corresponding to Er2TiO5 and two small intensity peaks corresponding to Er2O3 and Er(OH)x. This result indicates that the reaction of TiOx with Er atom forming an Er2TiO5 film suppresses the formation of Er(OH)x.

Figure
3a shows the C-V curves of the Al/Er2O3/TaN and Al/Er2TiO5/TaN capacitor devices. The Al/Er2TiO5/TaN capacitor exhibited a higher capacitance density than the Al/Er2O3/TaN one. In addition, the κ value of the Er2O3 and Er2TiO5 dielectric films is determined to be 13.7 and 15.1, respectively. Figure
3b depicts the current–voltage characteristics of the Al/Er2O3/TaN and Al/Er2TiO5/TaN devices. The Al/Er2TiO5/TaN device exhibited a lower leakage current than the Al/Er2O3/TaN device. This result is attributed to the formation of a smooth surface at the
oxide/channel interface.

The transfer characteristics of the a-IGZO TFT devices using Er2O3 and Er2TiO5 gate dielectrics were shown in Figure
4a. The VTH value of the Er2O3 and Er2TiO5 a-IGZO TFT devices is 1.5 and 0.39 V, whereas the Ion/Ioff ratio is 1.72 × 106 and 4.23 × 107, respectively. The moisture absorption of the Er2O3 film generates a rough surface due to the formation of Er(OH)x, thus causing degradation in the electrical characteristics. Furthermore, the Ioff current can be improved by bottom gate pattern to reduce the leakage path from the
gate to the source and drain. Furthermore, the μFE of the Er2O3 and Er2TiO5 TFT devices is 6.7 and 8.8 cm2/Vs. This result is due to the smooth roughness at the oxide-channel interface
[15]. The subthreshold swing (SS) of the Er2O3 and Er2TiO5 TFT devices is 315 and 143 mV/dec, respectively. The titanium atoms can effectively
passivate the oxygen vacancies in the Er2TiO5. The effective interface trap state densities (Nit) near/at the interface between the dielectric and IGZO were estimated from the SS
values. By neglecting the depletion capacitance in the active layer, the Nit can be calculated from the relationship
[6]:

(1)

where q is the electronic charge; k, the Boltzmann's constant; T, the temperature; and Cox, the gate capacitance density. The Nit values of IGZO TFTs using Er2O3 and Er2TiO5 gate dielectrics are about 6.92 × 1012 and 2.58 × 1012 cm−2, respectively. Figure
4b shows the output characteristics of the a-IGZO TFT devices using the Er2O3 and Er2TiO5 gate dielectrics. As is seen, the driving current increases significantly for the
Er2TiO5 dielectric material. This outcome may be attributed to the higher mobility and smaller
threshold voltage.

To explore the reliability of an a-IGZO transistor, the dc voltage was applied to
the high-κ Er2O3 and Er2TiO5 a-IGZO TFT devices. Figure
5a shows the threshold voltage and drive current degradation as a function of stress
time. The voltage stress was performed at VGS = 6 V and VDS = 6 V for 1,000 s. The shift in threshold voltage and the degradation in drive current
are associated with the trap states in the dielectric layer and the interface between
the dielectric film and channel layer
[16]. The large VTH shift (1.47 V) of the Er2O3 TFT can be due to more electrons trapping near/at the interface between the Er2O3 and IGZO layer
[6], whereas the low VTH shift (0.51 V) of the Er2TiO5 TFT device may be attributed to the reduction of the trapped charge in the film.
With increasing VGS, interface states are substantially generated, which are normally regarded to be
Er dangling bonds (=Er•), originating from the dissociation of weak Er-OH bonds at
the oxide/channel interface. The dissociation of Er-OH bonds under dc stressing is
proposed to be associated by the electrons in the oxide surface as follows:

(2)

Figure 5.Threshold voltage and drive current degradation and structural model. (a) Threshold voltage shift and current drive degradation as a function of stress time
for high-κ Er2O3 and Er2TiO5 a-IGZO TFT devices. Structural model of the (b) Er2O3 surface and (c) Er2TiO5 surface.

The physical model to be presented is based on the structure of the Er2O3 and Er2TiO5 surfaces, as schematically depicted in Figure
5b,c, respectively. Briefly speaking, during dc stress, hydroxyl ions (OH–) are released from the erbium hydroxide (Er-OH) by breaking the Er-OH bonds. The
electrons in the oxide have gained enough energy from the applied gate and drain voltages.
They collide with strained Er-O-Er or Er-O-Ti bonds to generate trapped charges in
bulk oxide, causing a threshold voltage shift. On the other hand, a-IGZO TFT with
the Er2O3 dielectric has a larger drive current degradation than that with the Er2TiO5 one. The hygroscopic nature of RE oxide films forming hydroxide produces oxygen vacancies
in the gate dielectric, leading to a larger flat-band voltage shift and higher leakage
current
[11]. The incorporation of Ti into the Er2O3 dielectric film can effectively reduce the oxygen vacancies in the film.

Conclusions

In conclusion, we have fabricated a-IGZO TFT devices using the Er2O3 and Er2TiO5 films as a gate dielectric. The a-IGZO TFT incorporating a high-κ Er2TiO5 dielectric exhibited a lower VTH of 0.39 V, a larger μFE of 8.8 cm2/Vs, a higher Ion/Ioff ratio of 4.23 × 107, and a smaller subthreshold swing of 143 mV/dec than that of Er2O3 dielectric. These results are attributed to the addition of Ti into the Er2O3 film passivating the oxygen vacancies in the film and forming a smooth surface. Furthermore,
the use of Er2TiO5 dielectric film could improve the stressing reliability. The Er2TiO5 thin film is a promising gate dielectric material for the fabrication of a-IGZO TFTs.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

FHC designed the experiment, measured the a-IGZO TFT device data, and drafted the
manuscript. JLH provided useful suggestions and helped analyze the characterization
results. YHS performed the experiment and measured the electrical characteristics.
YHM helped in the technical support for the experiments. TMP supervised the work and
finalized the manuscript. All authors read and approved the final manuscript.

Acknowledgment

This work was supported by the National Science Council (NSC) of Taiwan under contract
no. NSC-101–2221-E-182–059.